This invention relates to apparatus and method for detecting a security threat within a given area. More particularly, it relates to a detection system wherein the spectra of microwave signals reflected from targeted reflectors and of locally detected seismic signals are compared with predetermined spectral signal patterns or "signatures" to detect the presence of a security threat.
Many systems are known for detecting the presence of a security threat. However, these systems typically are expensive and/or are prone to generating false alarm signals. In addition, most known systems are limited in range and often may be circumvented by disabling a portion of the system, masking the RF signals, counterfeiting RF signals, etc.
For example, U.S. Pat. No. 4,274,089 to Giles discloses a merchandise security system in which a passive tag is attached to a piece of merchandise. An RF signal is transmitted into the detection zone with a given frequency. A receiving antenna picks up RF signals generated in the detection zone and provides an alarm signal whenever RF signals of a specific frequency are detected in the zone. The tag on the merchandise contains circuitry for receiving the transmitted signal at frequency f, doubling its frequency, and retransmitting a signal of frequency 2f. When the receiving antenna picks up the retransmitted 2f signal, an alarm signal is generated. Such a prior system apparently was limited in effectiveness to 1-3 meters (see column 4, lines 35 and 50) and is inherently subject to false alarms e.g. since the receiving antenna will produce an alarm signal whenever a signal with the doubled frequency is received). Another RF transmitter or an inadvertent reflective device inside the detection zone can also cause a false alarm. In addition, since the RF signal transmitted into the detection zone is not transmitted with any directivity, it is possible for this RF signal to be reflected by any reflector within the zone. Thus, if more than one tag is present in the zone, the system only detects one security threat.
Another known security detection system is described in U.S. Pat. No. 4,302,846 to Stephen et al. Stephen discloses a marker tag detection system similar to that described in Giles. However, Stephen transmits signals of two different frequencies into the detection zone and the tag reflects a signal having a frequency approximately midway between the two transmitted frequencies. The intermodulation characteristics of the two frequencies are said to enable the Stephen device to detect not only tag presence but also tag position within the zone. However, this marker tag detection system suffers from the same drawbacks as discussed with reference to Giles, above.
A surveillance system for detecting a security threat within a room is discussed in U.S. Pat. No. 4,382,291 to Nakauchi. Nakauchi transmits ultrasonic pulses into a room and then receives stores superposed digitized representations of the pulse reflections from objects and walls in the room. A "safe" reflected pattern is received and stored. Whenever "non-safe" objects or persons are within the room, the new reflected signature pattern is compared with the "safe" pattern and an alarm is generated if they do not match. However, such a system may suffer severe problems of false alarming. If any object in the room is caused to move, the system may generate an alarm. In addition, the presence of pets, insects, open windows (air circulation and/or blowing curtains) may also cause a false alarm.
Another alarm system for a room is described in U.S. Pat. No. 4,092,636 to Shepherd, Jr. In Shepherd, Jr., a microwave generator provides a directional beam to a microwave reflector mounted on a window or other vertical surface within the room. The beam is reflected back to the detector along a line-of-sight normal to the reflector. When the reflector is raised, broken, or altered in any way, or when the reflected beam is broken, an alarm is generated. Such a system may be limited in that a separate microwave generator appears to be provided for each reflector. In addition, this system may also be subject to false alarming. Vibration or movement of the reflector, the presence of pets or insects in the room, or blowing curtains may, for example, initiate a false alarm.
Some examples of further possibly relevant prior systems can be found in:
U.S. Pat. No. 3,518,546--Augenblick et al (1970) PA1 U.S. Pat. No. 3,938,125--Benassi (1976) PA1 U.S. Pat. No. 3,990,065--Purinton et al (1976) PA1 U.S. Pat. No. 4,139,844--Reeder (1979) PA1 U.S. Pat. No. 4,303,910--McCann (1981) PA1 U.S. Pat. No. 4,334,214--Satou et al (1982) PA1 U.S. Pat. No. 4,352,098--Stephen et al (1982)
From the above discussion, it is apparent that there is a continuing need for an inexpensive area security detection system that has a high probability of intruder detection, minimum false alarm probability, and which cannot be easily circumvented.